In the past, there has been widely known an internal combustion engine which is provided with an upstream side exhaust purification catalyst in an exhaust passage of an internal combustion engine and is provided with a downstream side exhaust purification catalyst in the exhaust passage at the downstream side in the direction of flow of exhaust with respect to the upstream side exhaust purification catalyst (for example, see PTL 1). In this internal combustion engine, the unburned gas (unburned HC, CO, etc.) or NOx, etc., which is not removed by the upstream side exhaust purification catalyst is removed by the downstream side exhaust purification catalyst. As a result, it becomes possible to suitably remove the unburned gas and NOx in the exhaust gas which is exhausted from the internal combustion engine.
For example, in the device which is described in PTL 1, when the output voltage of the downstream side oxygen sensor is at the high side threshold value or more and the state of the upstream side exhaust purification catalyst is an oxygen deficient state, the target air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst is made a lean air-fuel ratio. Conversely, when the output voltage of the oxygen sensor at the downstream side is at the low side threshold value or less and the state of the upstream side exhaust purification catalyst is the oxygen excess state, the target air-fuel ratio is made the rich air-fuel ratio. According to PTL 1, it is considered that due to this, when in an oxygen deficient state or oxygen excess state, it is possible to return the state of the catalyst quickly to a state between these two states (that is, state in which catalyst stores suitable amount of oxygen).
Further, when the output voltage of the downstream side oxygen sensor is at the low side threshold value or less, exhaust gas of an air-fuel ratio leaner than the stoichiometric air-fuel ratio (hereinafter referred to as “lean air-fuel ratio”) flows out from the upstream side exhaust purification catalyst. Therefore, in this case, exhaust gas which contains NOx flows out from the upstream side exhaust purification catalyst. On the other hand, when the output voltage of the downstream side oxygen sensor is at the high side threshold value or more, exhaust gas of an air-fuel ratio richer than the stoichiometric air-fuel ratio (below, referred to as “rich air-fuel ratio”) flows out from the upstream side exhaust purification catalyst. Therefore, in this case, exhaust gas containing unburned gas flows out from the upstream side exhaust purification catalyst. When in this way exhaust gas containing NOx or unburned gas flows out from the upstream side exhaust purification catalyst, these components in the exhaust gas are removed at the downstream side exhaust purification catalyst.